This thread got me to thinking... I've been in the insulated glass (IG) business going on 30 years now and at the first plant I ran we used to ship IG units all over the country from our home base in Chicago.  We had customers that we shipped units to for installation in the Denver area and other locations in the Rockies... if we simply sealed their IG units as we normally did for local installations, they would explode somewhere along the way as they gained altitude.  Consequently we had to provide a breather tube to allow the unit to equalize its internal pressure as it was moved into its high altitude final home.  Early on in the IG business they would use a piece of 1/8" aluminum tubing that was left open and included instructions to crimp it shut upon delivery.  This would allow the internal gases to vent without using up the desiccant, as the primary direction of flow was out of the unit until it arrived.  Somewhere along the way someone discovered that if you used a very small capillary tube (SS hypodermic needle stock) with a .010 ID opening it would still allow the pressure to equalize but for some reason the moisture vapor from the ambient air's humidity wouldn't make it into the unit. Consequently, once we started using this hypodermic tubing for breathers we were able to eliminate the requirement to squeeze them shut during installation.  We had units that remained in service for 10 years and more with an open hypodermic breather to the atmosphere and they never failed due to desiccant failure.  It makes me wonder if we wouldn't experience a similar phenomenon with our fuel tanks by using a hypodermic breather instead of a 1/4" vent line.  It would provide enough "breathing capacity" to allow the heating/cooling air encapsulated in the fuel tank to expand and contract as required, but maybe without sucking in the moisture vapor.  I know, some of you out there are going to say that's impossible, and I agree... I don't understand the physics of it, all I know is that it worked in another application, and wonder if there might be some crossover.  There's always something new to think about, isn't there?

 <marv>



Ernest Christley <echristl@cisco.com>:

"""
 Sealing your tank will slow the process down (a good thing), and only the
smallest of holes is necessary to keep from blowing your tanks.   I don't have
the plans with me to compute the exact area, but just 1psi of pressure will
put over 600lbs of force on the face of your tank (and about the same on the
bottom and back).  Please, drill a hole in your cap.  Use some micro-bits and
make it as small as you can, or screw it on very loosely so the air can push
out (or in) around the sides.  This may also save your bacon on the day that
your forget to remove it (Murphy says that you will eventually).
"""